Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce light olefin(s) such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds.
The petrochemical industry has known for some time that oxygenates, particularly alcohols, are convertible in the presence of molecular sieve catalysts into light olefins. Molecular sieves are porous solids having pores of different sizes such as zeolites or zeolite-type molecular sieves, carbons and oxides. The most commercially useful molecular sieves for the petroleum and petrochemical industries are known as zeolites, for example aluminosilicate molecular sieves. Zeolites in general have a one-, two- or three-dimensional crystalline pore structure having uniformly sized pores of molecular dimensions that selectively adsorb molecules that can enter the pores, and exclude those molecules that are too large.
Typically, molecular sieves are formed into molecular sieve catalyst compositions to improve their durability, control reactivity and improve cost-effectiveness in commercial conversion processes. Molecular sieve catalyst compositions are formed by combining a molecular sieve and a matrix material usually in the presence of a binder. Matrix materials, such as clays, are typically effective in reducing overall catalyst cost, acting as thermal sinks to assist in shielding heat from the catalyst composition for example during regeneration, densifying the catalyst composition, increasing catalyst strength such as crush strength and attrition resistance, and in controlling the rate of conversion in a particular process. The purpose of the binder is hold the matrix material to the molecular sieve.
In a typical MTO reaction system, undesirable byproducts may be formed through side reactions. For example, metals forming conventional reactor walls may act as catalysts in one or more side reactions. If the methanol feedstock contacts the metal reactor wall at sufficient temperature and pressure, the methanol may be converted to undesirable methane and/or other byproducts. Additionally, ethylene and propylene may react with steam at elevated temperatures and in the presence of certain metals to undesirably form acetaldehyde and acetone byproducts.
Byproduct formation in an MTO reactor is undesirable for several reasons. First, increased investment is required to separate and recover the byproducts from the desired light olefins. Additionally, as more byproducts are formed, less light olefins are synthesized. That is, the production of byproducts is undesirable because methanol feed is consumed to produce the byproducts. Further, although the relative concentrations of metal catalyzed side reaction byproducts are generally quite low, the total amount of byproducts produced on an industrial scale can be enormous. Thus, it is desirable to decrease or eliminate the synthesis of byproducts in an MTO reaction system.
Pending U.S. patent application Ser. No. 10/175,285, which was filed on Jun. 19, 2002, the entirety of which is incorporated herein by reference, discloses a method and apparatus for reducing the amount of metal catalyzed side-reaction byproducts formed in the feed vaporization and introduction (“FVI”) system of an MTO reaction system. In an FVI system of a reaction system, the feedstock is at least partially vaporized by one or more heating devices, is passed through feed lines to a feed introduction nozzle or nozzles, and is introduced into the reactor. Specifically, according to the '285 application, the temperature of at least a portion of the FVI system and/or of the feedstock contained therein is monitored and/or maintained below about 400° C. The temperature can be maintained in the desired range by jacketing at least a portion of the FVI system, such as at least a portion of the feed introduction nozzle, with a thermally insulating material or by implementing a cooling system.
Pending U.S. patent application Ser. No. 10/274,739, filed Oct. 21, 2002, the entirety of which is incorporated herein by reference, also is directed to reducing the formation of metal catalyzed side-reaction byproducts in the FVI system. Specifically, the '739 application discloses a method and system for reducing the formation of metal catalyzed side-reaction byproducts formed in the FVI system of an MTO reaction system by forming and/or coating one of more heating devices, feed lines or feed introduction nozzles of/with a material that is resistant to the formation of metal catalyst side reaction byproducts.
It has now been discovered that the presence of certain metals in the matrix material of a molecular sieve catalyst composition may exacerbate the formation of metal catalyzed side-reaction byproducts in an MTO reaction system. Thus, a need exists for reducing the formation of metal catalyzed side-reaction byproducts in an MTO reaction system caused by matrix materials in molecular sieve catalyst compositions.